Excessive alcohol consumption and illicit drug use are leading causes of death in the United States. Because a great majority of individuals with substance use disorders (SUDs) receive no treatment for their addictions, increasing SUD treatment access is one of the great public health challenges facing the US health care system. A project is proposed to evaluate the effects of the Patient Protection and Affordable Care Act (ACA) on rates of Medicaid and private health insurance enrollment as well as SUD treatment among adults with SUDs. The results will help state and federal health care policy makers gauge the early effects of the ACA on SUD treatment patterns and focus policies and programs on groups who have persistently low levels of health insurance and high levels of unmet need for SUD treatment. The specific aims of the project are to: 1) assess change in Medicaid and private insurance enrollment following ACA implementation among adults with SUDs; 2) determine whether among adults with SUDs, the ACA Medicaid expansion and private insurance subsidy provisions are associated with increases in SUD treatment and whether such increases are mediated by Medicaid and private insurance enrollment; and 3) explore associations between ACA implementation and likelihood of receiving specific SUD treatment services, sustained treatment, and treatment for alcohol and drug use disorders. We will address these aims by merging data from 1) the National Survey on Drug Use and Health (2010-2017), an annual face-to-face survey of approximately 70,000 state and national representative respondents that includes in depth information on the prevalence and treatment of SUDs; 2) the National Survey of Substance Abuse Treatment Services that includes detailed information on substance abuse treatment service capacity at the county level; 3) federal block grant funding levels to the states for substance abuse treatment and prevention services; 4) county resource characteristics from the Area Resource File; and 5) state-level participation in the ACA Medicaid expansion. A well-established conceptual model will guide selection of need, predisposing, and enabling control variables at the individual, county, and state levels. The analytic plan will model ACA implementation in a manner that is sensitive to ACA implementation period and target population of the ACA provisions by separately considering their effects on poor/near poor adults and middle income adults with SUDs. This project will generate a national assessment of the early effects of ACA on health insurance enrollment and treatment of adults with SUDs. In addition to the effects of the ACA on overall SUD treatment, the project will yield vital new information on how the legislation has influenced use of specialty inpatient and outpatient services, general outpatient services, and acute services for SUDs and its effects on populations of special interest including low income persons, young adults, females, and ethnic/racial minorities.